The IEEE 802.11 standards organization plans to formulate a IoT standard that is based on a 2.4 GHz/5 GHz frequency band, and basic features of the Wi-Fi IoT standard are low power consumption and a long distance. For the low power consumption feature, a possible method is using a low power (Lower Power, LP) wake-up radio (wake-up radio, WUR) on a Wi-Fi IoT device side. The wake-up radio is also referred to as a wake-up receiver (wake-up receiver, WUR). The wake-up radio is used as a unified name in descriptions of the present invention. Currently, a study group (Study Group) has been founded for a WUR standard of the IEEE 802.11, and a task group (Task Group) of the study group may be named 802.11ba. In other words, 802.11ba may be an official name of the IEEE 802.11 WUR standard.
The WUR means that an LP-WUR interface is introduced on a basis that a conventional Wi-Fi interface (802.11 main radio, 802.11 main radio) is configured on a device, as shown in FIG. 1. The 802.11 main radio is usually in an off mode, and only when receiving a wake-up signal from an LP-WUP, the 802.11 main radio is activated and then performs data communication with an AP. An LP-WUP of a STA remains in a receiving state or intermittently enters a receiving state. When receiving a wake-up frame (Wake-up Packet, also referred to as a wake-up frame) from the AP, the LP-WUR sends a wake-up signal to the 802.11 main radio, to wake up the 802.11 main radio that is in the off mode. Logically, an AP side actually includes an 802.11 main radio and a WUR module. However, for current 802.11 standards, the 802.11 main radio is usually an OFDM wideband transmitter, but a WUR wake-up signal is a narrowband signal. To reduce costs and make a structure simple, the OFDM wideband transmitter may be used to generate a narrowband. WUR wake-up signal. For example, some subcarriers of an OFDM signal are unoccupied, and a signal is transmitted only on narrowband corresponding to a WUR wake-up signal, to generate a narrowband signal. This is an example in which the OFDM wideband transmitter is used to generate a WUR narrowband signal. Therefore, the AP side in the figure includes only one module. It should be specially noted that in specific implementation of the AP side, the 802.11 main radio and the WUR module may be alternatively implemented separately. In addition, either of the AP and the STA in FIG. 1 has only one antenna, so that the 802.11 main radio and the WUR module may share a same antenna when using carriers in a same frequency band (for example, 2.4 GHz), to reduce costs and simplify a device structure. However, when the 802.11 main radio and the WUR module use carriers in different frequency bands, different antennas should be configured for the 802.11 main radio and the WUR module. For example, the 802.11 main radio uses the 5 GHz frequency band, and the WUR module uses the 2.4 GHz frequency band. In this case, the 802.11 main radio and the WUR module should correspond to different antennas.
The STA consumes less power by using the WUR instead of the 802.11 main radio to receive a signal. This is mainly because receiving and decoding of a wake-up frame are much simpler than receiving and decoding of a conventional 802.11 frame. The wake-up frame usually uses a modulation scheme that is easy for demodulation by a receiving device, for example, on-off-keying (on-off key, OOK) modulation. The OOK modulation is used as an example, and the receiving device determines, based on whether there is energy, information carried in a received signal. For example, if there is energy, the information is 1; or if there is no energy, the information is 0. Because a sending device performs OFDM, BCC/LDPC, and the like on the conventional 802.11 frame, correspondingly, the receiving device needs to perform complex signal processing operations such as FFT and FEC decoding, and these operations need to consume a lot of energy.
The 802.11 main radio of the STA in FIG. 1 may be alternatively another communications interface, for example, an LTE communications interface. A module for data communication is collectively referred to as a main communications module or a main communications interface (main radio), for example, an LTE module or a Wi-Fi module. A module for device wake-up is collectively referred to as a wake-up RF module or a wake-up RF interface (WUR).
A manuscript [11-16-0341-00-lrlp-low-power-wake-up-receiver-follow-up] puts forward a specific PFDU design of a wake-up frame, as shown in FIG. 2. An L-STF, an L-LTF, and an L-SIG are an 802.11 legacy preamble (legacy preamble. L-Preamble for short) part, are sent in a bandwidth of 20 MHz (or an integer multiple of 20 MHz) in an OFDM mode, and are used for backward compatibility, so that a conventional Wi-Fi device can determine that a current packet is a Wi-Fi packet, and therefore select a corresponding channel to listen to a CCA decision threshold. If the backward compatibility is not considered, the L-STF, the L-LTF, and the L-SIG may not exist. A payload (WUR Payload) part of the wake-up frame uses a modulation scheme that is easy for demodulation, for example, OOK modulation (specifically, such as ASK), and may be transmitted in narrower bandwidth, for example, a 2 MHz channel, a 4 MHz channel, or a 5 MHz channel (minimum bandwidth of conventional Wi-Fi is 20 MHz), so that the receiving device consumes less energy. The WUR payload includes a wake-up preamble and a MAC part. The wake-up preamble is similar to an STF, an LTF, and an SIG in conventional Wi-Fi, and is used for synchronization, AGC, channel estimation, control information indication, and the like. The MAC part is similar to a MAC part of a conventional Wi-Fi frame, and further includes a MAC header (MAC Header), a frame body (Frame Body), and a frame check sequence (FCS). Simple channel coding may be performed on the MAC part in a mode such as repetition code, spreading code, or Manchester code, to improve reliability. However, channel coding may alternatively not be performed Because a function of the wake-up frame (also referred to as a wake-up packet) is relatively simple, the frame body part may alternatively not exist. The wake-up preamble includes a specific sequence, and the WUR of the STA does not receive the preceding legacy preamble part but directly detects the specific sequence to identify a start of the wake-up frame. When receiving the wake-up frame and detecting an identifier (a unicast, multicast broadcast address) of the WUR of the STA in the MAC part of the wake-up frame, the WUR of the STA sends a wake-up signal to the 802.11 main radio. The wake-up preamble may further include a wakeup-signal (Wakeup-Signal, WU-SIG) field that is used to carry a length of the MAC part, a used modulation and coding scheme, and the like. In addition to OOK, the WUR payload part may alternatively use another modulation scheme that is easy for demodulation, for example, FSK.
The foregoing PPDU structure shown in FIG. 2 is only an example of a PPDU carrying a wake-up frame. Another structure may be alternatively used, provided that the PPDU can be received by the WUR interface. A PPDU that can be received by the WUR interface is collectively referred to as a WUR PPDU. The WUR PPDU may be used to carry not only a wake-up frame but also another frame that may be received by the WUR interface, for example, a synchronization frame used for WUR synchronization.
It should be specially noted that a sending device of a wake-up frame may be an AP, and a receiving device is a terminal device equipped with a WUR, for example, a mobile phone or a sensor; or a sending device of a wake-up frame may be a terminal device, for example, a mobile phone, and a receiving device is another terminal device equipped with a WUR, for example, a smartwatch or a smart band; or a sending device of a wake-up frame may be a terminal device, for example, a mobile phone, and a receiving device is an AP equipped with a WUR; or a sending device of a wake-up frame may be a terminal device, for example, a smartwatch or a smart band, and a receiving device is a terminal device equipped with a WUR, for example, a mobile phone. In short, a sending device of a wake-up frame needs to have a capability of sending a WUR PPDU, and a receiving device needs to be equipped with a WUR interface, to receive the WUR PPDU. For ease of description, in this application, an AP represents a sending device of a WUR PPDU, and a STA represents a receiving device of the WUR PPDU, but the AP and the STA do not represent specific product forms of the sending device and the receiving device.
If a WUR of a STA remains in an activated state for a long time, obviously, a lot of power is consumed. A trade-off method is that the WUR intermittently enters the activated state. A time window in which the WUR of the STA is in the activated state is referred to as a wake-up window (Wakeup window). Appearance of the wake-up window should be regular, so that an AP can know a time when the WUR of the STA can receive a wake-up frame. The wake-up window of the WUR of the STA may be referred to as a wake-up window of the STA for short. Such a working mode of the WUR of the STA is also referred to as a duty cycle (duty cycle) mode. For example, the WUR is in the activated state in 2 ms in every 100 ms. To be specific, a duty cycle period is 100 ms, and a length of the wake-up window is 2 ms, as shown in FIG. 3. When the AP needs to send data to the STA, the AP may send a wake-up frame in the wake-up window of the STA, to wake up an 802.11 main radio of the STA. Certainly, the wake-up window may alternatively not be introduced. In other words, the WUR of the STA is always in a listening state, so that the AP can wake up the STA at any time. This helps reduce a wake-up delay, but a disadvantage is that the STA consumes more energy.
When the WUR of the STA uses the duty cycle mode shown in FIG. 3, a wake-up frame sent by the AP needs to be in the wake-up window of the STA, so that the wake-up frame can be received by the STA, thereby waking up the STA. This means that the WUR of the STA needs to be synchronized with the AP. In other words, the AP needs to periodically send a synchronization frame in a WUR PPDU format. Otherwise, a clock drift caused by a crystal oscillator difference between the STA and the AP makes the AP incapable of estimating a location of the wake-up window of the STA. Such a wake-up frame sending manner based on synchronization may be referred to as synchronous wake-up. It is assumed that the length of the wake-up window is 2 ms. Estimated based on clock drift precision of a crystal oscillator of a current Wi-Fi device, a maximum period of sending a WUR synchronization frame by the AP is about a few seconds, so that it can be ensured that the STA is synchronized with the AP. When data from the AP is relatively frequent, a synchronous wake-up manner achieves a relatively good effect. However, if the AP only needs to send cached data to the STA after a long time, the synchronous wake-up manner causes the AP to send a large quantity of unnecessary WUR synchronization frames to maintain synchronization with the STA. This causes the AP to consume extra power. In addition, because a synchronization frame consumes a relatively long time (about hundreds of microseconds), sending of a large quantity of synchronization frames occupies many media resources, thereby reducing media resource utilization. Especially in a scenario in which BSSs are densely distributed, a plurality of adjacent APs need to send WUR synchronization frames, and consequently, a channel is full of a large quantity of WUR synchronization frames. This greatly reduces media utilization and seriously affects communication of conventional Wi-Fi (or a main communications interface).
A solution to overcome a problem that media utilization is reduced because the AP periodically sends a synchronization frame in synchronous wake-up is: The AP does not periodically send a WUR synchronization frame, but sends a plurality of wake-up frames in succession when needing to send cached data to the STA, to wake up the STA.
As shown in FIG. 4, because an AP does not know a location of a wake-up window of a WUR of a STA, when data of an associated STA arrives at the AP, the AP contends for a channel and then sends wake-up frames in succession. It should be noted that “sending wake-up frames in succession” herein means that the AP sends a plurality of wake-up frames, but the AP needs to contend for a channel before sending each wake-up frame, and a time resource between two adjacent wake-up frames may still be used by another device or be used by the AP to perform transmission with another STA. A time interval Δt between adjacent wake-up frames should not exceed a length of the wake-up window of the STA, to ensure that these wake-up frames do not miss the wake-up window of the STA in one duty cycle period T, thereby minimizing a delay of a wake-up process. When receiving a wake-up frame from an associated AP in the wake-up window of the STA, the STA immediately wakes up a main communications interface of the STA and sends a wake-up acknowledgement message such as a PS-Poll (Power Saving Poll, power save poll) frame to the AP by using the main communications interface.
In comparison with synchronous wake-up, a manner that is shown in FIG. 4 and in which no synchronization frame needs to be sent and the AP sends a series of wake-up frames only when needing to transmit data may be referred to as asynchronous wake-up. Although the AP needs to send a plurality of wake-up frames in the asynchronous wake-up manner, for a scenario in which data is not frequently sent, a quantity of these wake-up frames is far less than a quantity of periodic synchronization frames in the synchronous wake-up manner, and in other words, fewer channels are occupied. Therefore, in the scenario, asynchronous wake-up occupies less media, and saved resources may be used for communication of a Wi-Fi interface (or a main communications interface), thereby improving media utilization.
Because a WUR PPDU uses low-speed modulation such as OOK, even if the WUR PPDU carries a little data, one WUR PPDU occupies a very long time. It is estimated that transmission of one WUR PPDU may last for hundreds of microseconds, for example, 500 μs. If the WUR PPDU uses spreading code or another coding scheme to improve transmission reliability, a transmission time of the WUR PPDU is longer. It is assumed that a length of a wake-up window of a WUR is 2 ms, and a length of a WUR PPDU carrying a wake-up frame is 500 μs. In an asynchronous wake-up process, 25% time resources are occupied by the wake-up frame, and the entire process may last for hundreds of milliseconds (on a same order of magnitude as a duty cycle period). Therefore, there is a problem that media occupation is high, and during the time, communication of Wi-Fi (or a main communications interface) is still affected seriously. When BSSs are dense, the problem is more serious. The BSS is a network system including an access point and a device associated with the access point.